Modulation system



April 13, 1937.

M. G. cRos Y MODULATION SYSTE'M 1 Origirial Filed Sept. 19, 1931 v 12 Sheets-Sheet 1 INVENTOR MURRAY 6. CROSBY ATTORNEY Patented Apr. 13, 1937 v UNITED STATES PATENT OFFICE MODULATION SYSTEM Murray G. Crosby, Riverh ead, N. Y., assignor to Radio Gorporation of America, a corporation of Delaware Original application eptember 19, 1931, Serial No.. 583,725. Divided and this application m y 14, 1935. Serial No. 21.343

12 Claims. (01. its-+111) cordance with the message waves to be trans-- mitted. This modulation may refer to 'a variation in phase, frequency or amplitude of a high is frequency carrier wave where the variation ocours in accordance with signal impulses such as may becaused by sound or electrical waves of a relatively low frequency. a 5

' Heretofore, phase modulation has been accomplished by somewhat complicated circuit arrangements which are inserted in the transmitting, ap-

paratus. .These arrangements, it has been found, are rather expensive and complex,,and require careful maintenance.

25 Accordingly, a primary object of this invention is to secure an improvement'in the method of and apparatus for modulating electrical waves. A more specific object is to provide; a simple and efiicient phase modulation arrangement which 30 may 'be connected to any existing transmitter with very few changes.

According to one particular embodiment of this invention, these objects are attained by utilizing capacitive and inductive couplings between the l modulating device and the output of the carrier current amplifying apparatus. In aspecific embodiment, these couplings may take the characteristics of a transmission line over which the wave energy to be phase modulated is propa 40 gated. Varying the values of either type of reactance in the line varies the wave propagation and consequently its phase.

Various features of this invention reside in the different circuit arrangements employed for cou- 45 pling the modulating device to the transmitter apparatus-and will appear in the subsequent detailed disclosure.

Referring to the drawings:' 'Fig. 1 is a schematic diagram of a coinplete 50 transmitting system employing an artificial line.

Phase modulation is attained in this circuit by varying the capacity per-unit lengthof the artificial line; Figs. 2, -3 and 4 are views illustrating various 65 specific circuit schemes for connecting the artiflcial line of Fig. 1 to the other portions of the system.

Fig. 1 illustrates a transmitting system adapted for phase modulation wherein the distributed constants of an artificial-line are varied in accordance with the signal wave. A source of wave energy l of any nature such as, for example, a

crystal or long line control generator, is coupled byway of a transmission circuit included in the rectangle A to a load circut of any nature shown for'purposes of example as an antenna. A limiting device and an amp1iiier and a frequency multiplier may, if desired, be interposed between the line included in A and the output circult or the utilization point. An artificial line shown in box A, is connected to the source l by leads a and b. The output end of the transmission line 20 is coupled by leads d and e to the load circuit directly or by way of said limiter, power amplifier and frequency multiplier. Elements of the line are also connected by induct-, ances 2| to a lead cwhich in turn is connected to the means for controlling a characteristic of the .line. This means will be described in detail hereinafter.

In this particular arrangement glow discharge tubes I9 are utilized as a variable condenser and form part of the distributed capacity of the line. These tubes l9 may be neon tubes, 7 cathode ray tubes or any suitable type of tube wherein an electron stream may be varied to vary the capacity between the electrodes of' the tubes and the metallic plate intermediate said electrodes. With no current through the tube the capacity of the tube will only consist of the capacity between the electrodes and the metallic plate. The application'of a potential between the electrodes of the tube will cause a current to flow between the electrodes and a glow to strike which will, in turn, act as a conducting material between the electrodes. A variation of thepotential across the tube will vary the glow within the tube with a consequent variation in capacity of the tube. Thus, the variation in impedance in theplate circuit of modulator tube II will aifect the glow tubes and vary their capacity in accordance with the signal wave in ,an obvious manner. A plurality of radio frequency choke coils 2i, 2| of high impedance to the carrier fre ofthe carrier wave transmitted, over the arti- I ficial line by. changing the constants of the line.

The impedance of the plate circuit of tube I I may I be controlled in any manner as. for example, by

applying controlling potentials thereto from a source l6 by way of transformer I! connected with a control electrode of tube ll.

One advantage of this arrangement is that the length ofv the line may be increased to increase the amount of phase deviation with signal, thus obtaining a decrease in the percentage of variation of the variable distributed constants and consequently obtaining a more linear modulation. Any amplitude modulation caused in this circuit which might be undesirable may be removed in the system by employing limiters or over-loaded amplifiers whose output is limited to a definite value regardless of how much the input is increased. An ordinary vacuum tube amplifier will serve this purpose if it is over-loaded in such manner that its output does not increase in proportion to its input. Such limiting devices are well known in the art and therefore need not be illustrated here. If desired, frequency multipliers may be used to multiply the phase displacement of the modulated wave.

Another advantage of this particular arrangement is that an increase in phase deviation may be obtained by lengthening the artificial line. In such case, frequency multiplication will not be required.

Figs. 2, 3 and 4 illustrate different modulator units which may be employed to replace the apparatus'in box A of Fig. 1 in the complete transmitting system.

Referring to Fig. 2, there is shown a glow tube 2| in the form of a variable condenser within which an artificial line is helically wound. If desired this line may be wound around the outside of the tube in very close proximity to the tube. The capacity of the line to ground is varied by changing the glow tube current, thus causing phase modulation of the carrier current impressed on the artificial line. I

In Fig. 3 the effective inductance per un length of the artificial line is modulated by modulating the plate impedance shunted across the inductances 22, 22 which are coupled to the artlficial line at various points. In this manner the velocity of the carrier wave propagated along the artificial line is varied in accordance with the variation of the signal wave with a consequent modulation in phase of the carrier.

In Fig. 4 is an arrangement whereby the plate impedance of the modulator tube isserially connected with the capacity of the line to ground. A variation of this plate impedance effectively varies the capacity of the line to ground. The phase modulated wave energy produced in the modulators of the passing signals and appearing at d and e may be utilized directly or may be passed through an amplitude limiter of the overloaded tube type before using. If desired, the wave energy may be increased in frequency in I0 and also amplified before utilization.

It should be understood, of course, that this invention is not limited to the specific embodiments disclosed herein, but that various different organizations may be employed which embody the principles of this invention without departing from the spirit and scope thereof.

What I claim is:

1. A phase modulation system comprising a generator of constant frequency wave energy, an output circuit, a transmission line of considerable length as compared to the wave length of said wave energy comprising series inductanceand shunt capacity connected between said generator and said output circuit and means connected to said line for varying the effective electrical length thereof in response to signals to be transmitted.

2. Apparatus as claimed in claim 1, in which said last named means cooperates with the series inductance of said line to vary the effective series inductance of said transmission line at signal frequency.

3. Apparatus as recited in claim 1 in which said last named means cooperates with the shunt capacity of said line to vary the effectiveshunt capacity of said line at signal frequency.

7 4. Apparatus'as claimed in claim 1, including mea'nsfor rendering the energy in said output circuit of substantially constant amplitude.

5. A communication system for transmitting message waves, comprising a source of wave energy of carrier wave frequency, an outputcircult for saidsystem, an artificial line of considerable length relative to the wave length of said waveenergy comprising impedance elements intermediate said source and'said output, and means for varying the effective length of said artificial line in accordance with said message wave whereby the phase of thecurrent passing through said lines is varied in accordance with the message waves transmitted.

6. A communication system for transmitting message waves, comprising a source of wave energy of carrier wave frequency, an amplifying device, an artificial line of considerable length relative to the wave length of said wave energy consisting of inductive and capacitive elements intermediate said source and said device, and a modulating element for varying the impedance of either the inductive or capacitive elements of said line in accordance with said message wave whereby the phase of the current passing through said'line is varied in accordance with the message waves to be transmitted.

7. A system as defined in claim 6 wherein said capacitive elements comprise at least one glow tube.

8. A device for relaying carrier frequency oscillations and for modulating the phase of the oscillations relayed at signal frequency comprising a source of carrier frequency oscillations, an in-.

ductance connected to one terminal of said source, a plurality" of reactance elements con: necting points on said inductance to the other terminal of said source, and means for varying the value of said reactances at signal frequency to-thereby vary the phase of theoscillations impressed on said inductance by said source comprising a thermionic tube having its input electrodes energized at signal frequency. and its anode electrode connected to said reactances.-

9. In a phase modulation system, a source of wave energy of substantially constant frequency, a source of modulating potentials, a utilization circuit, an artificial line having distributed inductive reactance and distributed capacitive reactanceconnecting said source of wave energy to said utilization circuit, and an electron discharge amplifier having input electrodes coupled to said source of modulating potentials and having output electrodes connected in effect in shunt to at least one of said distributed reactances to vary the value thereof at the frequency of said modulating potentials, and thereby vary the ve-. locity of the wave energy along said line to said utilization circuit to thereby vary the phase of said energy reaching said utilization circuit at signal frequency.

10. A phase modulation system as recited in v 9,077,988 claim 9 wherein said distributed minivan:

actance comprises'the capacity between the glow discharge betweenvthe electrodes of a discharge device and a plateadjacent'the path of said dis-- charge, and in-which the output electrodes of 10 able length 'as compared to the length of the with the modulating potentials.

12. A-phase modulation system as recited in claim 9 wherein an amplitude limiting device is i 3 wave energy or said source of wave energiv to thereby insure considerable phase deviation of the wave traveling in said line in accordance interposed between said artificial line and said utilization circuit.

.MURRAY acriosmr. 

